More importantly, this is UK-based employment, and consequently, unless specifically available in the contract, overtime would not generally be available in this scenario.
Maybe in places like USA which has very poor employment law. However in the EU there isn't really that distinction, and employment law applies to all employees.
Not true. Software developers are mostly exempt, but there are some that will be non-exempt from overtime based on their salary. If the person makes less than $455 a week as salaried or less than $27.63 an hour as an hourly employeed then they will be non-exempt (http://www.dol.gov/whd/regs/compliance/fairpay/fs17e_computer.htm). However, I have a feeling it would be hard to find somebody paid at that rate.
I have a developer working for me who HR has decided is overtime-eligible, and he makes more like 3x that.
If I understand the law correctly (e.g., http://www.itstime.com/may2006.htm) having pay above the $455/week threshold is only one of the conditions for being overtime-exempt; the other conditions have to do with extent of job responsibility.
I've not heard of OSHA being involved in employer IP issues, nor in any software engineering environment.
Destroying something the company claims as their IP would subject someone to some serious criminal and civil liability.
Also, if the guy didn't like his job, he'd probably find another one. I get the sense he wants to keep this one and maintain a civil employment.